This invention relates in general to systems for annealing ion implantation damage and to activate ion-implanted dopants in semiconductor wafers, and in particular to a system for rapid isothermal annealing of the ion implantation damage and for activating dopants.
Since the early 1960's ion implantation has been adopted as a doping method for semiconductor wafers. To remove the damage induced by ion implantation and to activate the dopants electrically, diffusion furnaces have been used conventionally to anneal the wafers. In order to achieve uniform results, it is necessary for the conditions in the furnace to reach thermal equilibrium. For this reason, diffusion furnace annealing methods have innately long time constants. Thus, the heating time of semiconductor wafers in the diffusion furnace annealing methods cannot be less than 10 minutes; in general the annealing time in such methods is longer than 30 minutes. See, for example, "Rapid Wafer Heating: Status 1983" by Peter S. Burggraaf, Semiconductor International, December 1983, pp. 69-74 at p. 71, and Principle And Technology of Ion Implantation, Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing Press, Beijing, 1982.
Because of the lengthy annealing time required at high temperatures in diffusion furnace annealing, the dopants implanted become extensively redistributed. Thus, the PN junction depth in semiconductors processed conventionally is typically larger than 0.4 microns and the activation may be low for high-dose implanted dopants. With the development of very large scale integrated circuits (VLSI), it is frequently required to reduce the depth of a PN junction to as little as 0.2 microns or less and to greatly reduce the sheet resistance of the doped area. Conventional diffusion furnace annealing is therefore not suitable for VLSI manufacturing. It is therefore desirable to provide an alternative heating system which anneals the wafer faster than conventional diffusion furnace annealing methods and which achieves very high efficiency of activation, especially for high-dose implanted dopants.
For the reasons discussed above, different annealing methods have been developed since 1975 as alternatives to the conventional diffusion furnace system. Pulsed or scanning lasers and electron beams have been proposed for rapidly heating the wafer in order to accurately control dopant redistribution. While these systems may be useful for research and special applications, these systems may be too expensive or too complicated for industrial use.
Varian Associates, Inc. designed a vacuum transient annealing system, Varian model IA-200, as illustrated in FIG. 1. As shown in FIG. 1 a semiconductor wafer 2 is placed between tantalum reflectors 1 and 5. A resistance heated graphite element 4 is used as the radiant power source for heating the wafer. A multilayer tantalum shutter 3 is placed between the wafer and the heater to control the time for heating the wafer. Reflectors 1 and 5 improve heat efficiency by reflecting the radiation from the graphite element otherwise lost towards the wafer. The annealing system is placed in a metal container.
While the Varian system does activate more dopants in the annealing process in comparison to conventional furnaces, apparently it still cannot completely activate high-dose implanted dopants. For example, the activation is about 85% of 6.times.10.sup.15 /cm.sup.2 A.sub.s.sup.+ implanted silicon (Applied Physics Leters, 39, 604, 1981). Furthermore, in the Varian system, the metal container containing the components 1-5 is evacuated so that the annealing process is performed in vacuum. Thus, the wafer is heated by radiation only. Generally, it takes ten seconds for a wafer to be heated from room temperature to 1000.degree. C. While the Varian system heats the wafer at a rate much faster than that of conventional methods, it may be desirable to provide heating systems which can heat at a still faster rate for many VLSI applications to further reduce dopant redistribution. In addition, since surface thermal degradation easily occurs in vacuum, it appears that the Varian system cannot be used to anneal compound semiconductors.
Aside from the Varian system, three other commercial systems are available today from AG associate, Veeco/Kokusai and Eaton. These three systems use different configurations of high intensity lamps as energy sources. The Varian system and the three high intensity lamp systems are discussed by Burggraaf in the article referenced above. Thus, the rapid isothermal annealing methods have all used a form of radiation heat transfer instead of heat conduction or convection for rapidly heating the wafer.
Since the amount of dopant redistribution is strongly related to the annealing time, it is desirable to improve the efficiency of radiation heat transfer. As discussed in the Burggraaf article referenced above, wafer temperature uniformity and reduction of metal ion contamination are two of the most important requirements for designing rapid wafer heating systems.
The rapid isothermal annealing methods proposed by Varian and three other companies have achieved annealing times on the order of 10 seconds compared to 30 minutes for diffusion furnace annealing methods. In such rapid isothermal annealing methods, dopants redistribution can be accurately controlled. In addition, rapid isothermal annealing methods provide processing advantages over diffusion furnace annealing. Rapid isothermal annealing permits serial single wafer processing which has better repeatability and control characteristics than conventional diffusion furnace methods. Individual wafers are also easier to handle than batches. All these features are very important for VLSI processing. The serial individual processing permitted by rapid isothermal annealing methods lends itself to automation and blends easily with other cassette-to-cassette production equipment now available for wafer fabrication. Vendors have also suggested that rapid isothermal annealing methods use only 10% of the power required for diffusion furnaces and therefore lightens the load on the air-conditioning system in a processing area. Since wafers are processed in batches in diffusion furnaces, furnace annealing requires large clean rooms. In contrast, rapid isothermal annealing requires much less clean room area and less space.
While rapid isothermal annealing appears to give much improved results compared to conventional furnace annealing and appears poised to replace the latter as the annealing method of choice, the presently available systems are not entirely satisfactory. Thus, even though the Varian system can be used to heat a wafer from room temperature to 1000 degrees C. in about 10 seconds, some dopant redistribution will still occur so that it is desirable to reduce the annealing time further. Furthermore, in order to achieve uniform temperature throughout the surfaces of the wafer, the radiation field from a lamp or graphite element must be very uniform. This may be difficult to achieve. In addition, the conventional epitaxial system is also unsuitable for rapid isothermal annealing. In epitaxial system, the semiconductor wafers are placed directly on the graphite plate, which is heated by RF power and the wafers are heated through heat conduction. In this way the wafers and the graphite plate are heated and cooled simultaneously. Therefore the process cannot be quick enough for rapid thermal treatment. It is therefore desirable to provide new improved rapid heat treatment systems for annealing semiconductors in which such difficulties are alleviated.